This invention relates to direct-drive manipulators and force feedback devices. More particularly this invention relates to a direct-drive manipulator having three degrees of freedom and suitable for use as a pen-based force display for a virtual reality or telerobotic environment.
A manipulator can serve as an input device for controlling movement of a robot or other real or simulated device. A direct drive manipulator responds to operator manipulations using one or more actuators directly coupled to a load (on the output side) or an operator contact point (on the input side). This contrasts with an indirectly driven manipulator which responds to operator manipulations using one or more actuators indirectly coupled to the load or operator contact point through gears or other scaling devices.
A pen-based manipulator is characterized by an elongated member having a contact point for applying operator input forces and displacements. The manipulator enables an operator holding the elongated member to move the member within a workspace under the control of manipulator components (e.g., actuators, joints and links).
A force feedback device is characterized as an output device in which forces are applied to an operator holding the feedback device. In several telerobotic applications, for example, a master manipulator located away from a robot controls a slave manipulator located at the robot. The master manipulator serves as an input device for commanding movement of the robot via forces or displacements applied to the manipulator by an operator. In addition, the master manipulator sometimes serves as a force feedback device for exhibiting force sensations felt by the operator. In a force reflection application, for example, the forces encountered by the robot under control are reflected back to the operator at the manipulator to enable improved coordination of robot motion and dexterity. U.S. Pat. No. 5,072,361 (Davis et al.) discloses a force-reflective tele-operation control system in which a master station includes an actuator exhibiting force resistance to movement in a master link. Such resistance is to be comparable to the resistance encountered by a slave device so that movement of the master device by an operator tracks movement of the slave device being controlled.
A force feedback device also is referred to as a force display. In the computer field the term display refers to a visual output device upon which ephemeral images are shown. The display serves as a visual interface between an end user and a computer environment. An operator uses his visual sense to experience the images. Analogously, the term xe2x80x9cforce displayxe2x80x9d is coined to refer to an output device upon which ephemeral forces are exhibited. The force display serves as a force-reflective, haptic, kinaesthetic, or tactile interface between an operator and a real or simulated environment. The operator uses his sense of touch to experience the forces. The force display, however, typically is more than a display in that it also serves as an input device. A force display is a bidirectional mechanical interface through which an operator both applies and receives forces and displacements.
According to the invention, a direct-drive manipulator enables precision manipulation and force display at a control point. By using a pen-like or other end-effector an operator applies forces to the control point. The manipulator responds to the applied forces allowing movement of the control point within a workspace domain over three degrees of freedom. When combined with a controller implementing a control algorithm for a specific application environment, force sensations are reflected back to the control point to be experienced by the operator. Virtual reality, telerobotic, and other simulated, real or remote applications can be created to define a control algorithm. For example, a control algorithm may define immovable object shapes. An operator then is able to trace the virtual object shapes and feel the object boundaries. In another example, a control algorithm may define tissue having shape, texture and force resistance variables at different locations. An operator then may perform a virtual reality surgery, in which the control point is the cutting point of a scalpel. Depending on the position of the control point and force applied, the operator experiences the sensation of cutting though the virtual tissue. In a telerobotic application for remotely controlling a robot, the control algorithm is defined as a reflection of forces encountered by the robot. There are many other examples of a control algorithm that could be created to use with the manipulator. This invention is directed toward the manipulator with control point to be used as a force display.
To serve as a force display it is desirable that the manipulator have substantially no backlash, very low friction, and very low inertia. Backlash, friction and inertia detract from a natural feel of a control point. No matter how sophisticated a control algorithm, if the manipulator suffers from significant amounts of backlash, friction or inertia, then its use as a force display is compromised. It also is desirable that the manipulator have a high bandwidth so that high frequency force components can be displayed.
According to one aspect of the invention, direct drive actuators are used for the manipulator. Direct drive actuators have force display advantages over indirect drive actuators. Indirect drive geared actuators have unacceptable backlash and friction characteristics. Also, indirect drive actuators typically have lower bandwidth capability. Direct drive actuators are implemented here in a configuration for a control point having no backlash, very low friction and very high force generation bandwidth.
According to another aspect of the invention, a parallel actuator structure is used to control motion in a horizontal plane to achieve very low inertia of the control point. The parallel structure provides two degrees of freedom in a horizontal plane of motion.
According to another aspect of the invention, the parallel structure is a redundant structure including three chains in parallel coupled at the control point. Each chain includes an actuator and two links. Each actuator is positioned at a fixed origin. One end of an inner link is coupled to the actuator. An opposite end of the inner link is coupled to one end of an outer link. A joint is formed at the connection of the inner and outer links. The opposite end of the outer link is coupled to the control point. The three chains define a planar structure in which three actuators provide two degrees of freedom with redundancy.
The parallel redundant (3-chain) structure of this invention improves over prior parallel 2-chain structures by enabling a more uniform force capability throughout the manipulator workspace. In the workspace of a conventional 2-chain structure the maximum force that can be applied to a control point varies depending on where the control point currently is positioned. According to the 3-chain structure of this invention, however, the maximum force that can be applied is substantially uniform throughout the workspace.
According to another aspect of the invention redundant sensing also is performed by including a sensor at each actuator in the parallel structure. Redundant sensing enables more uniform high resolution position sensing throughout the workspace.
According to another aspect of the invention the end-effector defines an open tool interface point as the control point. An operator holds his finger, a pen-like tool or other tool to the interface point and applies forces or displacements to manipulate the control point. By providing an open interface there is no friction or backlash introduces by a device that would hold the tool tip in place at the control point. Similarly the operator can apply or remove the tool from the control point with a natural feel. For example if the tool is a scalpel in a surgical virtual reality application, the trainee physician can manipulate the scalpel as during a real operation, then apply the scalpel to the control point to perform a cutting/surgical maneuver. The operator therefore achieves a more realistic making and breaking of contact.
According to another aspect of the invention, a pair of rotational actuators rotate the parallel structure about an axis to approximate a linear motion along a third axis, and provide a third degree of freedom for the control point.
One advantage of the manipulator of this invention is that embodiments have substantially no backlash, very low friction, very low inertia and a very high force generation bandwidth enabling a natural feel when implemented as a force display. Another advantage is that embodiments have a substantially uniform force capability throughout there workspace. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.